media: pvrusb2: Fix incorrect variable used in trace message

media: pvrusb2: Fix incorrect variable used in trace message

The pvr2_trace message is reporting an error about control read
transfers, however it is using the incorrect variable write_len
instead of read_lean. Fix this by using the correct variable
read_len.

Fixes: d855497edbfb ("V4L/DVB (4228a): pvrusb2 to kernel 2.6.18")
Cc: stable@vger.kernel.org
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>